The present invention relates, in general, to a method for trimming an electrical component that is located within an integrated circuit, and more particularly to a method for trimming resistors by the use of metal migration augmented by laser heating.
The above referenced patent discloses a method of trimming the ohmic value of a resistor which has previously been fabricated on an integrated circuit wafer. The trimming process uses the mechanism of metal migration to reduce the value of a resistor until a precise, desired ohmic value is achieved. The trimming is typically accomplished by pulsing the resistor with high amplitude, small duty cycle current pulses. The approach is especially useful in a high volume manufacturing environment, having clear advantages over other methods such as laser trimming, link blowing or zener zapping. The resistors trimmed by metal migration (RTMM) occupy very little of the integrated circuit die area. Some resistor trimming methods devote 25% of the integrated circuit die area to trim and protection circuitry. When using the RTMM structure a test computer performs a trim algorithm rather than digital logic on the chip. The RTMM structure can be trimmed after the integrated circuit is packaged. The RTMM structure exhibits increased accuracy and resolution when compared to other methods of producing trimmed resistors. They can undergo a resistance change as small as 25 milliohms/pulse on a 25 ohm resistor. Finally, no additional expensive equipment is required. The part is trimmed at the same time as it is tested and using the same equipment.
Assuming the use of silicon for the substrate and resistor with aluminum metal being migrated to trim the resistor. A small duty cycle current pulse is applied to the RTMM. Electron momentum exchange causes movement of silicon and aluminum atoms in the direction of current flow. This phenomenon is commonly referred to as electromigration. Electromigration causes an aluminum filament to grow from the positive terminal to the negative terminal, dissolving silicon as it grows. As the filament grows the ohmic value of the resistor decreases. Repeated application of the current pulse further decreases the ohmic value of the resistor until a desired final value is obtained.
A junction isolated resistor is limited in the maximum voltage it can withstand by the breakdown limits of the junction. This limits the maximum current which can be passed through the resistor for a particular value of resistance. In practice this serves to limit both the maximum value of resistance and the maximum sheet resistivity which can be used for an RTMM. The high current required can also cause particles or other contaminants to move damaging the structure of the semiconductor die. The widespread heat produced by the trimming current can cause cracks in the silicon dioxide layers in the vicinity of the resistor. Finally annealing of the migrated metal is required after trimming to achieve a stable resistance value. Typically this is achieved by passing a lower current through the resistor than is required for metal migration. The level of this current and the exact duration of the current is hard to control adequately. There is a need for an improved method of trimming resistors by metal migration which extends the RTMM capability to allow higher valued resistors, more stability after trimming, and use with a low voltage process.